The present invention relates to a device for monitoring a modulation frequency of an optical signal, and in particular a device for monitoring a microwave or millimetric modulation frequency.
Proposals have already been made for hybrid transmission systems using optical fibers to transmit signals which are modulated at frequencies normally used for radio transmissions, i.e. mainly microwave or millimetric frequencies.
In this respect, reference can be made to the following two publications, for example:
J. F. Cadiou, D. Tanguy, E. Penard, P. Jaffrxc3xa9, H. Schmuck, E. Vergnol, D. Mathoorasing, xe2x80x9cHybrid fiber-radio systems in the mm-wave range: A comparison between available optical sourcesxe2x80x9d, International Topical Meeting on Microwave Photonics, 5MWP""98, Princeton, N.J., pp. 59-62, 1998; and
E. Vergnol, F. Devaux, D. Tanguy, E. Penard, xe2x80x9cIntegrated lightwave millimetric single-sideband source: Design and issuesxe2x80x9d, Journal of Lightwave Technology, Vol. 16, No. 7, pp. 1276-1284, 1998.
One of the main difficulties encountered with such hybrid systems lies in the need to be able to stabilize over time the oscillator which modulates the optical signal.
Modulation frequency drift must be prevented so as to avoid any interference between the various carriers.
An object of the invention is thus to propose a device making it possible to monitor a modulation frequency of an optical signal, and in particular a microwave or millimetric modulation frequency.
Optical filters with a photorefractive crystal have been known for a long time.
They work on the following principle.
When a correctly-oriented photorefractive crystal has applied thereto firstly a write optical signal at a given wavelength xcexe and secondly an optical signal to be filtered, a holographic grating is created on the crystal which reflects the signal to be filtered at a filter wavelength xcexf such that:
xcexf=xcexe(1xe2x88x92[sin (xcex1)/n]2)xc2xd
where n is the refractive index of the crystal and where xcex1 is the angle between the controlling light signal and the optical signal to be filtered.
A photorefractive crystal optical filter of that type is described in particular in the Applicants"" French patent application published under the No. 2,739,508.
The invention provides a device for monitoring a modulation frequency of an optical signalxe2x80x94in particular a device for monitoring the modulation frequency of an optical signal modulated at a microwave or millimetric frequencyxe2x80x94which device comprises a photorefractive crystal, a photodetector, and means for applying to said crystal firstly at least a fraction of the modulated optical signal and secondly a write optical signal for photoinducing a Bragg grating on said crystal, the orientations of the modulated optical signal and of the write optical signal relative to said crystal being such that at a given wavelength of the optical signal the photodetector detects an intensity for the signal reflected by the crystal that reaches a maximum when the optical signal is modulated at the modulation frequency to be detected.
Advantageously, the device includes means for deriving the write optical signal from the modulated optical signal.
In particular, the write optical signal is preferably a portion of the modulated optical signal.
The invention also provides a device for controlling the frequency of an oscillator whose output signal is used to modulate an optical signal. This device comprises a servo-control loop which comprises:
firstly a monitor device of the above-specified type receiving as input at least a portion of the optical signal modulated by the oscillator; and
secondly means for correcting the frequency of the oscillator as a function of the output signal from the photodetector of said monitor device.
The invention also provides a hybrid transmission system comprising emitter means, modulator means for modulating the signal output by the emitter means, an oscillator for generating a microwave or millimetric frequency signal for controlling the modulator means, and an optical fiber line over which the modulated signal is transmitted. This system further comprises a device of the above-specified type for controlling the frequency of the oscillator.
The invention also provides a system for controlling the modulation frequency of an optical signal, characterized in that it comprises two monitor devices of the above-specified type, centered on respective monitoring frequencies on either side of the modulation frequency that the system is to control, and in that it also comprises a differential amplifier on whose inputs the outputs from the photodetectors of the two monitor devices are injected.
The invention also provides a hybrid transmission system comprising emitter means, modulator means for modulating the signal output by the emitter means, an oscillator for generating a microwave or millimetric frequency signal for controlling the modulator means, and an optical fiber line over which the modulated signal is transmitted, the system being characterized in that it further comprises a system of the above-specified type for controlling the frequency of the oscillator.
Other characteristics and advantages of the invention appear further from the following description.